Equipoised arm assembly

ABSTRACT

An equipoised mechanism for supporting an object has a first set of parallel arms pivotally mounted on a supporting bracket, a first supported bracket pivotally connected to the first set of arms at the ends thereof opposite the supporting bracket, a second set of parallel arms pivotally mounted on the first supported bracket, and a second supported bracket pivotally mounted at the ends of the second set of parallel arms, opposite the first supported bracket, to which a mounting assembly and supported object are attached. The arms and brackets define two parallelograms by which the first and second supported brackets are maintained in a predetermined orientation with respect to the supporting bracket; balance and equalization forces on the mechanism are provided by a pair of springs operably connecting the first set of parallel arms and two sets of parallel springs operably connecting the second set of parallel arms.

United States Patent 1 I Krogsrud 111 3,774,873 1 1 Nov. 27, 1973 Jens Krogsrud, Oslo, Norway [52] US. Cl. 248/280, 248/324 [51] Int. Cl. A471 5/00, A47g 29/02 [58] Field of Search 248/123, 280, 160, 248/292, 324; 240/73 BJ [56] References Cited UNITED STATES PATENTS 3,426,190 2/1969 Bobrick 248/281 X 2,090,439 8/1937 Carwardine.... 248/160 2,287,577 6/1942 Stava 248/280 1,078,577 11/1913 Fox 248/280 3,498,577 3/1970 Mehr 248/280 Primary Examiner.l. Franklin Foss Attorney-Harold L. Stults [57] ABSTRACT An equipoised mechanism for supporting an object has a first set of parallel arms pivotally mounted on a supporting bracket, a first supported bracket pivotally connected to the first set of arms at the ends thereof opposite the supporting bracket, a second set of parallel arms pivotally mounted on the first supported bracket, and a second supported bracket pivotally mounted at the ends of the second set of parallel arms, opposite the first supported bracket, to which a mounting assembly and supported object are attached. The arms and brackets define two parallelograms by which the first and second supported brackets are maintained in a predetermined orientation'with respect to the supporting bracket; balance and equalization forces on the mechanism are provided by a pair of springs operably connecting the first set of parallel arms and two sets of parallel springs operably connecting the second set of parallel arms.

10 Claims, 8 Drawing Figures PATENTED NOV 27 I975 SHEET 2 OF 2 1 EQUIPOISED ARM ASSEMBLY This invention relates to equipoise support mechanisms, and more particularly to an equipoise mechanism utilizing two sets of parallel arms forming a linkage assembly having counterbalance springs whichprovide an equilibrium or balance condition throughout a range of movement of the arms for supporting unitary objects such as lamps and small television sets.

, An object of this invention is to provide improved counterbalance and equipoising support mechanisms. A further object of the invention is to support relatively heavy objects throughout a wide range of movement with minimum variations in the balancing forces. An-

. other object of the invention is to support an object with great precision and yet permit the object to be moved with relatively no resistance forces. A still further object of the invention is to support an object by an assembly which folds into a relatively small space when not in use. A still further object is to provide an equipoise support mechanism which is, sturdy in construction, dependable and safe in use, and which will perform satisfactorily even after long periods of use. These and other objects will be in part obvious and in part pointed out below.

According to an aspect of the invention, an object supporting mechanism is provided in which two independent linkage assemblies, each consisting of a set of parallel arms, maintain a supported object in a fixed orientation as it is moved to various positions. Springs associated with each set of arms are positioned so as to develop forces drawing the two arms in each set together thereby countering the force created by the weight of the supported object. In one set of arms, the spring force exactly balances the torque created by the weight of the supported object. The spring force in the other set, however, balances the torque created by the combined weights of the other set of parallel arms and the supported object. As the arms are rotated about their respective pivots to a horizontal position, the torque created by the supported object increases. However, the restoring force created by the springs also increases since the springs are stretched during arm rotation. This increase in spring tension exactly balances the increase in-torque created by relocation of the supported object so that the net torque about the arm pivots is maintained at zero. In order to ensure proper folding of the mechanism into a small space when not in use, separate pairs of pivot axes between the two sets of arms are provided which are spaced so as to permit folding of the two sets of arms to where they are adjacent. The connection points of the springs are so located as to not only provide maximum supporting torque when the mechanism is not in use, but also provide a reversed torque for holding one set of arms in a folded position when not in use.

Utilization of such an arrangement permits a wide area of possible object location. One example of such use is for supporting television sets in hospital rooms. Since each patient maybe required to lie in a different position than others, it is important that the television set be locatable in many positions in order to ensure proper viewing.

Since in the preferred embodiment of this invention relatively strong springs may be used, it is important to provide some additional safety feature. Safety is increased by utilizing an arm construction which eliminates gaps between the two arms of one set. Such construction also permits concealment of two of the springs associated with this set.

In the drawings:

FIG. 1 is a side view of one embodiment of the invention;

FIG. 2 is a side view of a portion of the embodiment of FIG. 1, moved to a second position;

, FIG. 3 is a top plan view of the portion of the invention shown in FIG. 2;

FIGS. 4 and 5 are sectional views respectively taken along lines 4--4 and 5-5 of FIG. 1; and

FIGS. 6, 7 and 8 are enlarged sectional views of the interconnecting pivot structures for the arm assembly shown in FIG. 1.

Referring to FIG. I of the drawings, an equipoise support mechanism 10, includes a first pair of channel arms 12 and 14, an arm-supporting bracket 20, a first and 18 and a second supported bracket 24. Armsupporting bracket 20 is formed by two similarly shaped flat parallel plates 19 which are attached to the opposite'sidesof a pivot block 21 (see FIG. 7), the lower portion of which is an integral elongated pivot pin 30. The entire mechanism 10 can therefore be pivoted about the vertical axis of pin 30 when the pivot is inserted in a complementary bracket or base (shown in dotted lines in FIG. 1).

Arms 12 and 14 are pivoted at their lower ends upon arm-supporting bracket 20 respectively at pivots 32 and 34. The ends of arms 12 and 14 opposite bracket 20 are connected to first supported bracket 22 through pivots 36 and 38, respectively. The distance between pivots 36 and 38 is equal to the distance between pivots 32 and 34 and the distance from pivot 32 to pivot 36 .is equal to the distance between pivot 34 and pivot 38.

formed by two parallel plates 41 each having an angu lar portion between two parallel bends 43 to accommodate the different width of arms 16 and 18 relative to arm 12. The other ends of arms 61 and 18 are connected to bracket 24 at their respective pivots44 and 46. The distance between pivots 44 and 46 is equal to the distance between pivots 40 and 42 and the distance from pivot 40 to pivot 44 is equal to the distance "-between pivot 42 and pivot 46. Thus, a second parallelogram is formed by these four pivots which, in this case, varies in shape as arms 16 and 18 are turned about their respective pivots, as shown in FIG. 2.

Bracket 24 is formed by two parallel plates45 each having an angular portion between two parallel bends 47 to accommodate the narrower width of an interface bracket 56 therebetween. Bracket 56 is formed from a single plate which is bent to form two parallel sides 51 and 53 (removed in FIG. 8) and a rounded lower portion 57.. Pins 54 and 55 secure bracket 56 to bracket 24. Rounded lower portion 57 is maintained by the parallel linkage assemblies in a constant vertical portion as the two sets of arms are moved. A pivot bracket 58 of known construction is attached to lower portion 57 and permits rotational movement of a supported object (such as the television shown in dotted lines in FIG. ll) about an axis parallel tothe axis of elongated pivot 30. Pivot bracket 58 also provides a transverse pivot 62 for swinging bracket 60 to which the supported object is attached.

The weight of a supported object is counterbalanced by a series of springs attached to the linkage assemblies. A first pair of parallel coil springs 26, located on opposite sides of arms 16 and 18 as shown in FIG. 3, are connected at one end to the ends of a post 50 mounted on arm 18, and at the other end to the ends of a post or pivot pin 48 located slightly above and to the left of pivot 40 on bracket 22 in FIGS. 1 and 6. A second set of parallel coil springs 28, also located on opposite sides of arms 16 and 18, are connected to the ends ofa post 52 mounted on upper arm 16, and to the ends of a post or pivot pin 54 located slightly below and to the right of pivot 46 on bracket 24 in FIGS. I and 8. Spring pairs 26 and 28 are selected to as to provide counterbalance forces within the probable weight range of the supported object.

Referring now to FIG. 4, positioned between arms 12 and 14 is a spring connecting member or rod 70, of square tubular construction having two hooks 72 and 74 at its ends formed from its wall 75. Rod 70 is freely pivoted on pivot 38 and is not otherwise physically attached to arm 14. A coil spring 66 has one end looped onto upper hook 74 and its other end looped onto a hook 82 which is attached to arm 12 by screw 80. A secondcoil spring 64 has one end looped onto the lower hook 72 and its other end attached to an adjustment bolt 76. Thehead of bolt 76 is seated in a flat portion 78 formed from a wall extension of arm 12 which is looped around pivot 32. Thus, spring 64 is operatively connected to pivot 32 and facilitates counterbalancing forces thereat. Spring 64, the only adjustable spring in the mechanism, can be adjusted to provide the proper counterbalance for the supported object and to balance the action of adjacent spring 66.

Across-section of arms 12 and 14 is shown in FIG. S.'Each of these arms consists of a single plate which is bent in a generally rectangular U-shape channel. The external width of arm 14 is slightly smaller than the internal .width ofarm 12 thus permitting arm 14 to nest into arm 12 in an unobstructed manner. As arms 12 and 14 are moved about their respective pivots, as the position of the supported object is changed, they are displaced relative to each other. However, at no point does the open section of arm 14 move past the open section of arm 12 and thus no gap ever exists between arms 12 and 14.

This construction also provides a concealed passageway for an electrical cord which may be required by the supported object. Such a cord may pass from the supported object up through the center of brackets 60 and 58, through brackets 56 and 24, through arm 52 and bracket 22, down through the center cavity between arms 12 and 14 and out of bracket through a horizontal hole 23 in the upper square portion of the pivot block 21.

As mentioned above, the lower spring 64 within arms 12 and14 is adjustable, whereas the adjacent spring 66 as well as the two pairs of parallel springs 26 and 28 are not. Adjustment of springs 64 can be made to compensate for variations in the weight of the supported object. The strength of the other springs are selected to provide counterbalance forces within acceptable ranges determined by probable usage. Attachment of springs 64 to the spring connection rod rather than to the adjacent wall of arm 14 ensures that spring 64 will be stretched to a maximum amount equal to the increase in distance between pivot 32 and pivot 38 as the two parallel arms 12 and I4 are moved. Spring 66 is stretched a proportionally smaller distance because of the closeness between its points of connection to arm 12 at hook 82 and pivot 38. Springs 64 and 66 exert opposite torques on the spring connection rod about pivot 38. However, since the points of connection of springs 64 and 66 to rod 70 are selected so that spring 66 has a longer lever arm about pivot 38 than does spring 64, the torque produced by spring 66 about pivot 38 is greater than that produced by spring 64 so that rod 70 is held firmly against the inside wall of arm 14 as shown in FIG. 4.

Arms I2 and 14 have two positions of maximum stability or limits of movement with respect to each other, both occurring when the arms are in a closed configuration with respect to each other. One of these positions is shown in FIGS. 1 and 4, wherein pivot pin 38 is engaged by the side edges of arm 12, so that no further counterclockwise movement of the arms can take place. The second position of maximum stability (not shown) occurs when arms 12 and 14 are lowered below the horizontal (in a clockwise direction as seen in FIG. 1) so that the parallelogram formed by the arms is reversed from that shown in FIG. 4 with pivots 32 and 36 positioned to the left of pivots 34 and 38, respectively, and pivots 34 and 36 respectively engage the edges of arms 12 and 14 and thus further movement in a clockwise direction is prevented.

Another feature of the invention is that arms 16 and 18 also have two positions of maximum stability or limits of movement. The first of these positions is shown in FIG. l and occurs when the two arms are in a generally vertical position touching each other. The second such position is when the arms are folded down below the horizontal and again touch each other.

Because of the locations of posts 48 and 54, an overcenter effect is created so that a counterclockwise re storing force is provided by springs 26 and 28 as arms 16 and 18 are rotated clockwise (direction defined in accordance with FIG. 1) until a point is reached in which posts 48 and 50 and pivot 42, as well as posts 52 and 54 and pivot 44, are in a straight line. This point is a null position on each side of which there is a reversal in restoring torque. As arms 16 and 18 are rotated clockwise past this point, springs 26 and 28 act to rotate arms 16 and 18 toward arms 12 and 14.

By the above described construction it is thus seen that the equipoise mechanism may be considered to be two independent parallelogram linkage assemblies; arms 12 andl4 and springs 64 and 66 providing support for bracket 22, the second parallelogram linkage assembly and the supported object, and the second parallelogram linkage assembly, consisting of arms 16 and 18 and springs 26 and 28, providing support for the supported object relative to bracket 22.

As a result of the configuration of the arms, as described above, and the location of their respective pivot points and spring connections, the linkage mechanism is stable at any position in which the arms are placed,

between their respective positions of maximum stability. Accordingly, the flexibility of the device, i.e., the number of positions in which it may be placed is substantial. For example, arms 12 and 14 may be placed in any position between their two extremes, and arms 16, 18 may then be varied or moved as desired to any position between their extremes in order to properly position the television set or other supported article. Similarly, the relative position of arms 16 and 18 may be adjusted and then the arms l2, l4 moved as desired. Of course, both sets of arms may be moved simultaneously until the desired position of the supported article is attained.

Moreover, at the positions of maximum stability of the lower arms 12, 14, an extremely stable stable condition is provided for the entire range of movements of the upper arm. Further, heavier objects may be supported on the arm assembly of the present invention than have heretofore been .able to be supported on equipoised mechanisms.

As many possible embodiments may be made of the apparatus of the above invention, all without departing from the scope of the invention, it is to be understood that all matter hereinabove set forth, or shownin the accompanying drawings is to be interpreted as illustrative and not in a limiting sense. What is claimed is:

1. In an equipoise mechanism of the character described, the combination of, a first pair of parallel arms of substantially the same length and comprising a first arm and a second arm, an arm-supporting bracket assembly comprising an elongated pivot and an armsupporting bracket, first-pivot means pivoting said first and second arms of said arm-supporting bracket respectively upon a first pair of parallel axes which are spaced from each other at a predetermined distance axially with respect to said elongated pivot, a first supported bracket which is positioned at the ends of said first and second arms opposite said arm-supporting bracket, second pivot means pivoting said first supported bracket respectively upon a second pair of parallel axes which are spaced from each other similarly to said first pair of parallel axes, a second pair of parallel arms of substantially the same length and comprising a third arm and a fourth arm, third pivot means pivoting said third and fourth arms on said first supported bracket respectively upon a third pair of parallel axes, asecond supported bracket which is positioned at the ends of said third and fourth arms opposite said first supported bracket, fourth pivot means pivoting said third and fourth arms on said second supported bracket respectively upon a fourth pair of parallel axes spaced similarly to said third pair of parallel axes, and a mounting assembly attached to said second supported bracket for holding an object, said arms and said brackets defining a linkage assembly forming two variable parallelograms by which said first and second supported brackets and said mounting assembly are maintained in a predetermined relationship with respect to said elongated pivot axis when said arms are moved; a spring connecting rod of shorter length than said second arm located between said first and second arms and pivotally mounted at one end about one axis associated with said second pivot means, an adjustable spring operably connected at one end to said first arm adjacent said arm-supporting bracket and at its other end to said spring connecting rod at the end of said rod opposite said second pivot means, a second spring attached at one end to said first arm at a point between the ends of said first arm and at its other end to a said spring connecting red at the end of said rod adjacent said second 6 pivot means, a first pair of coil springs positioned in parallel relationship upon the upposite sides of said third and fourth arms with one end of each of the first pair of parallel springs being attached to said first supported bracket along an axis remote from said second and third pairs of parallel axes and with the other end of each said first pair of parallel springs attached to said fourth arm, a second pair of springs positioned respectively upon the opposite sides of said third and fourth arms and substantially parallel to each other with one end of each of the second pair of parallel springs being attached to said second supported bracket along an axis remote from said fourth pair of parallel axes and with the other end of each said second pair of parallel springs attached to said third arm, said springs cooperating to provide balanced and equalized forces at the pivots to counterbalance the weight of a supported object.

2. A mechanism as described in claim 1, wherein said first arm has a longitudinally extending recess therein and said second arm is narrower than said first arm and fits within said recess to prevent any external gap between said first and second arms at any position of said linkage assembly.

3. A mechanism as described in claim 2, wherein said first and second arms are generally U-shaped in crosssection and open towards each other, and wherein the open side of said first arm defines :said recess receiving said second arm.

4. A mechanism as described in claim 1, wherein the axis at which said first pair of springs are connected to said first supported bracket and the axis at which said second pair of coil springs are connected to said second supported bracket are located to provide a point of maximum spring extension during rotation of said third and fourth arms about their respective pivot axes on each side of which the torque applied by the two pairs of springs is in opposite directions.

5. A mechanism as described in claim 1, wherein the second and third pivot means are positioned to permit folding of the linkage assembly to a position wherein said fourth arm is adjacent to said second arm.

6. In an equipoised object-supporting mechanism, the combination of, a first pair of adjacent parallel arms of substantially equal length and comprising a first arm and a second arm, an arm-supporting bracket, first pivot means pivoting said first and second arm at one end on said arm supporting bracket respectively upon first and second parallel axes which are spaced a first predetermined distance from each other, a first supported bracket which is positioned at the end of said first and second arms opposite said arm-supporting bracket, second pivot means pivoting said first supporting bracket respectively upon third and fourth parallel axes which are spaced said first predetermined distance from each other, a second pair of adjacent parallel arms of substantially equal length and comprisinga third arm and a fourth arm, third pivot means pivoting said third and fourth arms at one end on said first supported bracket respectively upon fifth and sixth parallel axes spaced a second predetermined distance from each other, a second supported bracket which is positioned at the end of said third and fourth arms opposite said first supported bracket, fourth pivot means pivoting said third and fourth arms on said second supported bracket respectively upon seventh and eighth axes which are spaced said second predetermined distance from each other, a mounting assembly attached to said second supported bracket for holding an object, said arms and said brackets defining a linkage assembly forming two variable parallelograms by which said first and second supported brackets and said mounting assembly are maintained within a predetermined range of relative relationships with respect to said armsupporting bracket when said object is moved, an elongated connecting member positioned between said first and second arms and extending along said second arm with one end pivoted to said second arm at said fourth axis and with a free end connecting portion positioned substantially intermediate said first and fourth axes, a first counterbalance spring connected at one end to said free-end connecting portion and at its other end at said first axis, a second counterbalance spring connected at one end to said first arm substantially intermediate said first and third axes and connected at its other end to said connecting member at a zone which is spaced radially from said fourth axis toward said second arm, whereby said second spring tends to swing said connecting member about said fourth axis toward the adjacent portion of said second arm, said springs cooperating to urge said arms toward each other and to provide counterbalancing forces, and a plurality of springs each of which is connected at one end to one of said supported brackets and at its other end to one of said third and fourth arms, all of said springs cooperating to provide counterbalancing forces for the support of said object.

7. The mechanism as described in claim 6 wherein said first and second axes are positioned to swing said first and second arms between first rest position wherein they extend substatially vertically upwardly from said axes and are adjacent each other and a second rest position wherein they extend substantially horizontally and are separated from each other the maximum distance,and wherein'said springs and said arms cooperate to exert forces which cooperate with the weight of said object to tend to hold said arms in each of said rest positions.

8. In an equipoised object-supporting mechanism, the combination of, a first pair of adjacent parallel arms of substantially equal length and comprising a first arm and a second arm, an arm-supporting bracket, first pivot means pivoting said first and second arm at one end on said arm supporting bracket respectively upon first and second parallel axes which are spaced a first predetermined distance from each other, a first supported bracket which is positioned at the end of said first and second arms opposite said arm-supporting bracket, second pivot means pivoting said first supporting bracket respectively upon third and fourth parallel axes which are spaced said first predetermined distance from each other, an elongated connecting member positioned between said first and second arms and extending along said second arm with one end pivoted to said second arm at said fourth axis and with a free end connecting portion positioned substantially intermediate said first and fourth axes, a first counterbalance spring connected at one end to said free-end connecting portion and at its other end at said first axis, and second counterbalance spring connected at one end to said first arm substantially intermediate said first and third axes and connected at its other end to said connecting member at a zone which is spaced radially from said fourth axis toward said second arm, whereby said second spring tends to swing said connecting member about said fourth axis toward the adjacent portion of said second arm, said springs cooperating to urge said arms toward each other and to provide counterbalancing forces.

9. In an equipoised object-supporting mechanism, the combination of an arm supporting bracket, a first pair of adjacent parallel arms of substantially equal length and comprising a first and a second arm, first pivot means pivoting said first and second arm at one end on said arm supporting bracket respectively upon first and second parallel axes, a first supported bracket positioned at the ends of said first and second arms opposite said arm supporting bracket, said second pivot means pivoting said first supporting bracket respectively upon third and fourth parallel axes, a second pair of adjacent parallel arms of substantially equal length and comprising a third and a'fourth arm, third pivot means pivoting said third and fourth arms at one end on said first supported bracket respectively upon fifth and sixth parallel axes, a second supported bracket positioned at the end of said third and fourth arms opposite said first supported bracket, fourth pivot means pivoting said third and fourth arms on said second supported bracket respectively upon seventh and eighth axes, said arms and brackets defining a linkage assembly forming two variable parallelograms, a first set of springs operatively connected between said first and second arms in a generally parallel and longitudinal spaced relationship with respect to each other, and at least a second pair of spring operatively connected to said third and fourth arms in parallel spaced relation with respect to each other, one of said springs in said second pair being connected at one end to said fourth arm and at its opposite end to said first supported bracket at a point remote from said fifth and sixth axes located to position said one spring from one side to the other of said first axis as said third and fourth arms move through a position wherein said fifth axis, said point remote from said fifth axis, and the point of connection of said one spring to said fourth arm all lie on a common straight line whereby said one spring acts with an overcenter effect" to reverse the restoring torque applied to said third and fourth arms.

10. A mechanism as defined in claim 9 wherein the other of said springs in said second pair is connected at one end to said third arm and at its opposite end to said second supported bracket at a point remote from said seventh and eighth axes located to position said other spring from one side to the other of said eighth axes as said third and fourth arms move through a position wherein said eighth axis, said point remote from said seventh and eighth axes and the point of connection between said other spring and said third arm all lie in a common straight line whereby said other spring also acts with an overcenter effect to reverse the restoring torque applied to said third and fourth arms.

a: a a 

1. In an equipoise mechanism of the character described, the combination of, a first pair of parallel arms of substantially the same length and comprising a first arm and a second arm, an arm-supporting bracket assembly comprising an elongated pivot and an arm-supporting bracket, first pivot means pivoting said first and second arms of said arm-supporting bracket respectively upon a first pair of parallel axes which are spaced from each other at a predetermined distance axially with respect to said elongated pivot, a first supported bracket which is positioned at the ends of said first and second arms opposite said arm-supporting bracket, second pivot means pivoting said first supported bracket respectively upon a second pair of parallel axes which are spaced from each other similarly to said first pair of parallel axes, a second pair of parallel arms of substantially the same length and comprising a third arm and a fourth arm, third pivot means pivoting said third and fourth arms on said first supported bracket respectively upon a third pair of parallel axes, a second supported bracket which is positioned at the ends of said third and fourth arms opposite said first supported bracket, fourth pivot means pivoting said third and fourth arms on said second supported bracket respectively upon a fourth pair of parallel axes spaced similarly to said third pair of parallel axes, and a mounting assembly attached to said second supported bracket for holding an object, said arms and said brackets defining a linkage assembly forming two variable parallelograms by which said first and second supported brackets and said mounting assembly are maintained in a predetermined relationship with respect to said elongated pivot axis when said arms are moved; a spring connecting rod of shorter length than said second arm located between said first and second arms and pivotally mounted at one end about one axis associated with said second pivot means, an adjustable spring operably connected at one end to said first arm adjacent said arm-supporting bracket and at its other end to said spring connecting rod at the end of said rod opposite said second pivot means, a second spring attached at one end to said first arm at a point between the ends of said first arm and at its other end to a said spring connecting rod at the end of said rod adjacent said second pivot means, a first pair of coil springs positioned in parallel relationship upon the upposite sides of said third and fourth arms with one end of each of the first pair of parallel springs being attached to said first supported bracket along an axis remote from said second and third pairs of parallel axes and with the other end of each said first pair of parallel springs attached to said fourth arm, a second pair of springs positioned respectively upon the opposite sides of said third and fourth arms and substantially parallel to each other with one end of each of the second pair of parallel springs being attached to said second supported bracket along an axis remote from said fourth pair of parallel axes and with the other end of each said second pair of parallel springs attached to said third arm, said springs cooperating to provide balanced and equalized forces at the pivots to counterbalance the weight of a supported object.
 2. A mechanism as described in claim 1, wherein said first arm has a longitudinally extending recess therein and said second arm is narrower than said first arm and fits within said recess to prevent any external gap between said first and second arms at any position of said linkage assembly.
 3. A mechanism as deScribed in claim 2, wherein said first and second arms are generally U-shaped in cross-section and open towards each other, and wherein the open side of said first arm defines said recess receiving said second arm.
 4. A mechanism as described in claim 1, wherein the axis at which said first pair of springs are connected to said first supported bracket and the axis at which said second pair of coil springs are connected to said second supported bracket are located to provide a point of maximum spring extension during rotation of said third and fourth arms about their respective pivot axes on each side of which the torque applied by the two pairs of springs is in opposite directions.
 5. A mechanism as described in claim 1, wherein the second and third pivot means are positioned to permit folding of the linkage assembly to a position wherein said fourth arm is adjacent to said second arm.
 6. In an equipoised object-supporting mechanism, the combination of, a first pair of adjacent parallel arms of substantially equal length and comprising a first arm and a second arm, an arm-supporting bracket, first pivot means pivoting said first and second arm at one end on said arm supporting bracket respectively upon first and second parallel axes which are spaced a first predetermined distance from each other, a first supported bracket which is positioned at the end of said first and second arms opposite said arm-supporting bracket, second pivot means pivoting said first supporting bracket respectively upon third and fourth parallel axes which are spaced said first predetermined distance from each other, a second pair of adjacent parallel arms of substantially equal length and comprising a third arm and a fourth arm, third pivot means pivoting said third and fourth arms at one end on said first supported bracket respectively upon fifth and sixth parallel axes spaced a second predetermined distance from each other, a second supported bracket which is positioned at the end of said third and fourth arms opposite said first supported bracket, fourth pivot means pivoting said third and fourth arms on said second supported bracket respectively upon seventh and eighth axes which are spaced said second predetermined distance from each other, a mounting assembly attached to said second supported bracket for holding an object, said arms and said brackets defining a linkage assembly forming two variable parallelograms by which said first and second supported brackets and said mounting assembly are maintained within a predetermined range of relative relationships with respect to said arm-supporting bracket when said object is moved, an elongated connecting member positioned between said first and second arms and extending along said second arm with one end pivoted to said second arm at said fourth axis and with a free end connecting portion positioned substantially intermediate said first and fourth axes, a first counterbalance spring connected at one end to said free-end connecting portion and at its other end at said first axis, a second counterbalance spring connected at one end to said first arm substantially intermediate said first and third axes and connected at its other end to said connecting member at a zone which is spaced radially from said fourth axis toward said second arm, whereby said second spring tends to swing said connecting member about said fourth axis toward the adjacent portion of said second arm, said springs cooperating to urge said arms toward each other and to provide counterbalancing forces, and a plurality of springs each of which is connected at one end to one of said supported brackets and at its other end to one of said third and fourth arms, all of said springs cooperating to provide counterbalancing forces for the support of said object.
 7. The mechanism as described in claim 6 wherein said first and second axes are positioned to swing said first and second arms between first rest position wherein they extend substatially vertically upwardly from said axes and are adJacent each other and a second rest position wherein they extend substantially horizontally and are separated from each other the maximum distance, and wherein said springs and said arms cooperate to exert forces which cooperate with the weight of said object to tend to hold said arms in each of said rest positions.
 8. In an equipoised object-supporting mechanism, the combination of, a first pair of adjacent parallel arms of substantially equal length and comprising a first arm and a second arm, an arm-supporting bracket, first pivot means pivoting said first and second arm at one end on said arm supporting bracket respectively upon first and second parallel axes which are spaced a first predetermined distance from each other, a first supported bracket which is positioned at the end of said first and second arms opposite said arm-supporting bracket, second pivot means pivoting said first supporting bracket respectively upon third and fourth parallel axes which are spaced said first predetermined distance from each other, an elongated connecting member positioned between said first and second arms and extending along said second arm with one end pivoted to said second arm at said fourth axis and with a free end connecting portion positioned substantially intermediate said first and fourth axes, a first counterbalance spring connected at one end to said free-end connecting portion and at its other end at said first axis, and second counterbalance spring connected at one end to said first arm substantially intermediate said first and third axes and connected at its other end to said connecting member at a zone which is spaced radially from said fourth axis toward said second arm, whereby said second spring tends to swing said connecting member about said fourth axis toward the adjacent portion of said second arm, said springs cooperating to urge said arms toward each other and to provide counterbalancing forces.
 9. In an equipoised object-supporting mechanism, the combination of an arm supporting bracket, a first pair of adjacent parallel arms of substantially equal length and comprising a first and a second arm, first pivot means pivoting said first and second arm at one end on said arm supporting bracket respectively upon first and second parallel axes, a first supported bracket positioned at the ends of said first and second arms opposite said arm supporting bracket, said second pivot means pivoting said first supporting bracket respectively upon third and fourth parallel axes, a second pair of adjacent parallel arms of substantially equal length and comprising a third and a fourth arm, third pivot means pivoting said third and fourth arms at one end on said first supported bracket respectively upon fifth and sixth parallel axes, a second supported bracket positioned at the end of said third and fourth arms opposite said first supported bracket, fourth pivot means pivoting said third and fourth arms on said second supported bracket respectively upon seventh and eighth axes, said arms and brackets defining a linkage assembly forming two variable parallelograms, a first set of springs operatively connected between said first and second arms in a generally parallel and longitudinal spaced relationship with respect to each other, and at least a second pair of spring operatively connected to said third and fourth arms in parallel spaced relation with respect to each other, one of said springs in said second pair being connected at one end to said fourth arm and at its opposite end to said first supported bracket at a point remote from said fifth and sixth axes located to position said one spring from one side to the other of said first axis as said third and fourth arms move through a position wherein said fifth axis, said point remote from said fifth axis, and the point of connection of said one spring to said fourth arm all lie on a common straight line whereby said one spring acts with an ''''overcenter effect'''' to reverse the restoring torque applied to said third and fourTh arms.
 10. A mechanism as defined in claim 9 wherein the other of said springs in said second pair is connected at one end to said third arm and at its opposite end to said second supported bracket at a point remote from said seventh and eighth axes located to position said other spring from one side to the other of said eighth axes as said third and fourth arms move through a position wherein said eighth axis, said point remote from said seventh and eighth axes and the point of connection between said other spring and said third arm all lie in a common straight line whereby said other spring also acts with an ''''overcenter effect'''' to reverse the restoring torque applied to said third and fourth arms. 